fix: calibrate UF-F3 PDF severity so must-fix means certain rejection (#82)

[YuZh98]

What

Calibrate the UF-F3 PDF-layer severity so must-fix means certain rejection. Previously every per-page body-size deviation emitted must-fix.

A deviating page is now must-fix only when:

• the document-wide modal body size is itself off 12pt (a global \fontsize override), or
• the page's body text is larger than 12pt (oversized text is never a tolerated float).

An undersized page on an otherwise-12pt document (a \footnotesize table or a \small figure sub-caption) is demoted to review.

Why

The old logic false-failed real dissertations the UF Graduate School accepts. The PDF check derives body_size as the per-page modal glyph size; on a page dominated by a float, that modal is the caption/cell font, not running body text. A confirmed-accepted submission with \footnotesize appendix tables and \small sub-captions was flagged with 5 must-fix items. must-fix must mean "the editorial office will reject this"; small caption/table fonts do not. Closes #82.

How it works

• New helper _document_body_size(pages) returns the document-wide modal body size, ties resolving toward 12pt then the smaller size (an ambiguous document is treated as conforming → its deviating pages route to review).
• _check_f3 emits must-fix when doc_deviates or oversized, else review.

Verification

• Full suite: 540 passing, coverage 93.74% (gate 70%), deprecation-strict clean, pre-commit clean.
• New tests pin: localized undersize → review; global override → must-fix; global undersize (10pt) → must-fix; oversized page on a 12pt doc → must-fix; tie-break → review; _document_body_size edge cases.
• The committed \fontsize{20} violation fixture still fires must-fix (oversized branch).
• Real-world: the originating dissertation now reports 0 must-fix, 5 review (was 5 must-fix).

Docs

• docs/uf-rules.md UF-F3 section documents the severity calibration.
• CHANGELOG.md [Unreleased] → Changed.